The ones with cams are simple and easy to use and works by inserting one of the cams and there's very little fuzz with settings. I think stitch length is adjusted with a side lever. The Panoramic and Professional have most cams and with a few extra large cams the other ones doesn't have. Those with cams have keyhole shaped too. There a few variants on these, even for slant shanks. The older black type have cast aluminium cams, the newer ones are plastic; both options are well within acceptable and there's very little problems with them. Mechanically the buttonholer are mostly metal, some have plastic covers and knobs but the internal is metal.
The ones with out cams can be adjusten to any size with in it's min-to-max range. I have the old black Singer version (green box), I think the slant shank version is in the burgundy box (Helen Howes have a picture of one), low shank in a green; but always double check with seller, because mixups are common. There is a bit more screws involved and a bit more test buttonholes, but the gradualy buttonhole size is nice. Once set for the correct size, you are ready to stitch one after the other. These have straight buttonholes only.
I don't think you need the cover plate for the 401g, since as you mention the needle plate can be lifted above the feed dogs. The videos on youtube shows them running with out the additional plate. I would think the slant shank version were made for this. Models like 301 can't lower the feed dogs, and I guess that's the reason the cover plate is often in the box sets. It's much the same with the straight stitch models. When buttonholers were new to me I had a feeling they behaved better with the cover plate, but it turned out to be all about thread tension, needle size, and presserfoot tension. I have only used them on my 201 and 99, but it's very much the same thing on any machine. The buttonholer and needle clamp do all the work.
Its' hard to recommend one or the others, they work very well and each have their advantages. I have both and are both keeping and using them.